Electrographic printing or copying apparatuses are known—see for example WO 98/39691 A1. In such a printing or copying apparatus, charge images of the images to be printed are generated by a character generator on a charge image carrier, for example a photoconductor belt. The charge image carrier is subsequently moved past developer stations, respectively one per color. Thus the developer with toner particles is transported to the charge image carrier. The toner particles transfer to the charge image carrier corresponding to the charge images on the charge image carrier and ink the charge image carrier. The toner images are transfer-printed to a printing substrate in the next step and are fixed thereon. The precise workflow of the printing method can be learned from WO 98/39691 A1, a content of which is herewith incorporated into this specification by reference.
A developer fluid having at least toner particles and carrier fluid can thereby be used to ink the charge images. Possible carrier fluids are, among other things, silicon oil or hydrocarbons. One method for such an electrophoretic liquid development in digital printing apparatuses is known from WO 2007/082791 A1, for example. A carrier fluid containing silicon oil with toner particles dispersed in it is thereby used as a developer fluid; charge control substances can additionally be added to the developer fluid.
The feed of the developer fluid to the charge image carrier can take place via an applicator; for example an application roller or developer roller or an application belt that moves the developer fluid past the charge image carrier. The developer fluid can be supplied to the applicator by, for example, an inking roller across a developer gap existing between the inking roller and the applicator. For this an electrical field can be generated across the developer gap between the inking roller and the applicator, via which electrical field the electrically charged toner particles are drawn to the applicator.
A sufficient electrophoretic mobility of the toner particles in the carrier fluid and a uniform layer of developer fluid on the applicator are significant for the development of the charge images. The mobility of the toner particles is thereby affected by their charge, wherein the charge can be adjusted via the concentration of charge control substances in the developer fluid. In one development principle in which the developer fluid consists exclusively of carrier fluid, toner particles and charge control substances, the electrical conductivity of the developer fluid depends on the concentration of the charge control substances in the developer fluid. This can be established via measurement. For example, if the electrical conductivity of the developer fluid should fall below a desired value during operation, the desired concentration can be corrected via addition of charge control substances and the mobility of the toner particles can be changed. A disadvantage of this method is that the mobility of the toner particles is only indirectly and integrally assessed, and therefore weakly charged toner particles (or toner particles discharged at surfaces) can also arrive at the applicator. Ionic contaminations, injections of charge carriers from boundary surfaces that have a difference potential relative to at least one adjacent surface, and runtime-conditional degradations of the toner particles can therefore lead to an unwanted change of the correlation between conductivity and corresponding change of the particle charge or the electrophoretic mobility. Such a modification of this correlation then leads to an incorrect regulation of the charge control substances that can imply a change of the toner behavior in the printing process and therefore a degradation of the print quality.